In this study we explored the relation between private speech and task mastery by using the microgenetic method to examine the language and performance of 13 children aged 4 and 5 as they gained expertise with a spatial, multi-step planning task across 6 sessions. Seven of the 13 children's performances across these sessions were characterized by a sharp change in performance between two consecutive sessions. Detailed microgenetic examinations of individual children suggested that abrupt improvements in performance were associated with a higher use of on-task private speech. More variability was noted in the problem-solving and private speech patterns of children who did not experience abrupt shifts in performance. Implications regarding the role of individual differences in the use of private speech during problem-solving are discussed. (Contains 2 tables, 6 figures and 3 notes.)